Alzheimer's disease is a progressive neurodegenerative disorder characterized clinically by cognitive decline with onset usually after the age of 60 years. Its major neuropathological features include senile plaques, many with abnormal neurites (neuritic plaques), neurofibrillary tangles within neuronal perikarya, and amyloid angiopathy. Several neurotransmitter systems are perturbed in Alzheimer's disease, with cholinergic deficiency being most prominent and associated with cell loss in the nucleus basalis of Meynert.sup.1.
Substantial clinical, pathological, biochemical, and genetic heterogeneity exists in Alzheimer's disease. Frequently, Alzheimer's disease patients exhibit extrapyramidal signs of Parkinson's disease.sup.2,3 and have coexisting neuropathological features of Parkinson's disease (AD+PD). These can include substantia nigra degeneration and Lewy bodies in the pigmented nuclei and nucleus basalis of about 20-40% of neuropathologically confirmed autopsy brains.sup.4,5. Cases displaying cortical Lewy bodies have been called "diffuse Lewy body disease".sup.6-10, and are increasingly recognized with and without concomitant Alzheimer's disease pathology. Thus, the clinical and neuropathological features of Alzheimer's disease and Parkinson's disease overlap extensively, suggesting that they may represent a spectrum of disease with related causal mechanisms.
Our recent investigations have suggested that oxidative phosphorylation (OXPHOS) defects may play a role in the pathogenesis of Alzheimer's disease and Parkinson's disease.sup.11,12. OXPHOS enzyme assays in Parkinson's disease brains have shown Complex I defects.sup.13-15 as well as systemic OXPHOS defects in platelets.sup.16 and skeletal muscle.sup.17,18. Similarly, Alzheimer's disease patients have shown Complex IV defects in platelets.sup.19 and abnormalities in mitochondrial respiration in neocortex.sup.20 and fibroblasts.sup.21. Treatment of normal human fibroblasts with the OXPHOS uncoupler CCCP (carbonyl cyanide m-chlorophenylhydrazone) results in a 10-fold increase in the proportion of cells reacting with an antibody to paired helical filaments and a 157-fold increase in cells reacting to the Alzheimer's monoclonal antibody-50.sup.22, suggesting a linkage between OXPHOS defects and Alzheimer's disease pathology. Abnormal mitochondria with paracrystalline inclusions, like those frequently encountered in the muscle from patients with mitochondrial DNA deletions and point mutations.sup.23, have been described in the brain of a patient with AD+PD pathology.sup.24 and patients with Alzheimer's disease.sup.25.
OXPHOS is composed of five enzyme complexes assembled from 13 mitochondrial DNA (mtDNA) and approximately 50 nuclear DNA subunits. Complex I (NADH:ubiquinone oxidoreductase) contains seven mtDNA coded subunits (ND1,2,3,4,4L,5,6); Complex III (ubiquinol:cytochrome c oxidoreductase) has one mtDNA coded subunit (cytochrome b); Complex IV (cytochrome c oxidase) has three mtDNA coded subunits (COI, COII, COIII); and Complex V (ATP synthase) has two mtDNA coded polypeptides (subunits 6 and 8). The mtDNA also codes for a complete set of tRNAs and the 12S and 16S rRNAs necessary for mitochondrial protein synthesis.sup.11,12,23.
The cytoplasmic location, high copy number, and elevated mutation rate of the mtDNA result in a unique mitochondrial genetics.sup.11,12,23. The mtDNA is maternally inherited.sup.26,27, and intracellular mixtures of mutant and normal mtDNAs (heteroplasmy) segregate during mitotic and meiotic replication.sup.28-31. Different tissues and organs rely on mitochondrial OXPHOS to different extents. Therefore, as OXPHOS declines because of increasingly severe mtDNA mutations, organ specific energetic thresholds are traversed, yielding variable clinical phenotypes.sup.31,32,33. OXPHOS enzyme activities decline with age.sup.34-36, concomitantly with the age-related accumulation of mtDNA damage in stable tissues.sup.37-40. This may accentuate inherited OXPHOS defects as individuals age, leading to clinical manifestations late in life.sup.11,12.
As a result of the quantitative aspects of mtDNA genetics, families harboring deleterious mtDNA mutations frequently show highly variable phenotypic expression among maternal relatives.sup.11,12. This is seen for both heteroplasmic mtDNA mutations which segregate between generations.sup.31,33 and homoplasmic mutations.sup.41-43, whose expression can be influenced by additional mtDNA mutations.sup.42,44-46, nuclear DNA alleles.sup.47, and/or environmental factors. OXPHOS diseases are further complicated by the fact that different mtDNA mutations can act alone or in synergistic groups to produce the same phenotype.sup.42. This is best exhibited in Leber's hereditary optic neuropathy (LHON), where the same disease has been associated with mutations in the Complex I ND1, ND2, ND4, and ND5 genes.sup.41,42,44,45,48-50, in the Complex III cytochrome b gene.sup.42,46, and in the Complex IV COI gene.sup.51.
MtDNA mutations which cause LHON fall into two categories: (1) nucleotide substitutions with low pathogenicity that exist as rare polymorphisms in the general population and increase the probability of expressing the disease phenotype.sup.42,44-46 and (2) nucleotide substitutions with high pathogenicity that cause maternally transmitted disease.sup.41,48-50. The low pathogenicity mutations typically cause sporadic disease within families and can be identified through phylogenetic analysis by the clustering of patients around specific mtDNA haplotypes.sup.42,44,51. Within these pathological mtDNA lineages, sequential mutations frequently accumulate, increasing the probability of clinical manifestations.sup.42,44. The highly pathogenic mutations frequently cause familial disease and occur randomly in the population in association with a variety of mtDNA haplotypes.sup.52.
Moreover, because of the continued importance of Alzheimer's disease and Parkinson's disease, there is an urgent need to diagnose as well as predict a predisposition to the diseases. This invention satisfies this need by demonstrating that defects in mtDNA are associated with Alzheimer's disease and Parkinson's disease and can be used to predict the likelihood of developing Alzheimer's disease and Parkinson's disease and used to diagnose Alzheimer's disease and Parkinson's disease.